Fix Cordova indents with new clean html option

[thewheat]

Fixes #1004

• Did this by adding a new option in CleanHtmlFilter and setting it in the Cordova scraper
• The indents are removed as they are a non breaking space character that gets removed by CleanHtmlFilter. The option allows bypassing the whitespace removal if it detects a non breaking space
• I'm open to other ways of doing this more cleanly

Existing unindented with new indented

Source indented with new indented

[simon04]

In order to not pollute the core filters with rarely used options, I've created lib/docs/filters/cordova/clean_html_core.rb which is used instead of lib/docs/filters/core/clean_html.rb

Updating cordova docs is blocked since the scraper needs to be adapted to changes on cordova.apache.org. I'll merge nevertheless.